MANILA -- From Singapore to the Philippines, Southeast Asian nations are spending big on submarines and other vessels to ramp up their naval capabilities amid rising tensions in the South China Sea.

The five major Southeast Asian countries have all increased defense spending by double digits in the five years through 2016, according to the Stockholm International Peace Research Institute. Indonesia and Vietnam have both seen a more than 60% boost. Seven countries in the region ranked among the world's top 40 arms importers last year, accounting for over 9% of global imports.

Singapore's Defense Ministry on Tuesday signed a contract to purchase two submarines from Germany's ThyssenKrupp Marine Systems. It had bought two of the same vessels in 2013. It is replacing older submarines in order to secure nearby sea lanes, as trade traffic through the Strait of Malacca grows.

The Republic of Singapore Navy held its inaugural International Maritime Review the day before. "The RSN had a humble beginning and started with only two wooden ships," President Tony Tan Keng Yam said at a parade. "Fifty years on, the RSN is now an advanced and integrated naval force comprising frigates, submarines, naval helicopters and other vessels."

Thailand's military government also recently approved the purchase of a non-nuclear submarine from China for 13.5 billion baht ($391 million at current rates), with plans to buy two more. It overrode domestic opposition, citing the need to increase maritime security.

From land to sea

Traditionally in Southeast Asia, the army has been the more influential military arm. But with the growth in maritime trade and Beijing's militarization in the South China Sea, countries are shifting their focus to nautical forces. Economic growth has expanded room for military spending, and Japan and other countries are also providing Southeast Asian nations with more defense equipment.

The Philippine Coast Guard dispatched a surveillance ship it received from Japan to Benham Rise earlier this month. China is believed to have conducted unauthorized exploratory activities by the formation, located 250km east of the Philippine island of Luzon.

Japan has provided three 40-meter surveillance vessels to the Philippines so far, and is scheduled to hand over seven more by 2018. It is also planning to provide two 90-meter vessels. Many of the Philippine Coast Guard vessels are older, and the country was forced to back down in the face of China's overwhelming might during a 2012 standoff in the disputed Scarborough Shoal. Manila is working to modernize its equipment, and increased military spending by nearly 40% between 2011 and 2016.

Meanwhile, Vietnam has purchased six Kilo-class submarines from Russia, the last of which was delivered to its key military port in Cam Ranh Bay in January. The U.S. also lifted its arms embargo on Vietnam last year for the first time since the Vietnam War, which means the Southeast Asian nation could start buying American arms. Outside the region, Taiwan launched a homegrown submarine program in March.

No country wants a military clash with China. But the growing naval presence could ultimately lead to an arms race in the region and raise the stakes in the South China Sea.